This invention relates to an alkali-free glass adapted to form a glass plate for use as a display substrate of a flat display, such as a liquid crystal display (LCD) and an electro luminescence (EL) display, a glass plate for use in an image sensor, such as a charge coupled device (CCD) and a contact image sensor (CIS), a solar cell, and the like. This invention also relates to a glass plate using the alkali-free glass.
Presently, use is widely made of a flat display, such as an LCD and an EL display. In particular, an electronic device such as an active matrix thin film transistor LCD (TFT-LCD) is small in thickness and low in power consumption and is therefore used in various applications, for example, in a car navigation system and a finder of a digital camera and, recently, in a monitor of a personal computer and a TV. As a display substrate of the flat display, a glass plate is widely used.
A TFT-LCD panel maker attempts to improve the productivity and to reduce the cost by providing a plurality of devices on a glass plate (blank plate) formed by a glass maker and cutting the glass plate to individually separate the devices as products. Recently in the applications such as the monitor of the personal computer and the TV, the device itself is required to be large in size. In order to obtain a plurality of such large-sized devices, there is a demand for a glass plate having a large area, for example, having a size of 1000×1200 mm.
On the other hand, a mobile apparatus, such as a mobile telephone and a notebook-type personal computer, is required to be light in weight so that it is conveniently and easily carried by a user. Consequently, the glass plate is also required to be light in weight. In order to lighten the weight of the glass plate, the reduction in thickness is effective. At present, the glass plate for the TFT-LCD typically has a thickness as very small as about 0.7 mm.
However, the glass plate having such a large size and such a thin thickness exhibits large sag due to its weight. This causes a serious problem in a production process as follows.
In the glass maker, the glass plate of the type is formed and then subjected to various steps such as cutting, annealing, testing, and cleaning. In these steps, the glass plate is loaded into and unloaded from a cassette having multi-deck shelves. In detail, the cassette is provided with the shelves formed on two inner surfaces on left and right sides or on three inner surfaces on left, right, and back sides. The glass plate is held in a horizontal position with its two opposite sides or three sides supported on the shelves. As described above, the glass plate of a large size and a thin thickness exhibits large sag. Therefore, when the glass plate is put onto the shelves of the cassette, a part of the glass plate is often brought into contact with the cassette or another glass plate to be broken. On the other hand, when the glass plate is taken out from the shelves of the cassette, the glass plate may be heavily shaken to become unstable. In the display maker also, a similar cassette is used and similar problems occur.
The sag of the glass plate due to its weight varies in direct proportion to the density of a glass or a glass material forming the glass plate and in inverse proportion to the Young's modulus. Therefore, in order to suppress the sag of the glass plate, the specific modulus given by the ratio of Young's modulus/density must be increased. To this end, the glass must have a high Young's modulus and a low density. Even with the same specific modulus, the glass having a lower density, i.e., relatively light per unit volume allows the glass plate having the same weight to be increased in thickness. The sag of the glass plate varies in inverse proportion to a square of the thickness. Therefore, such increase in thickness of the glass plate is very effective in suppressing the sag. The low density of the glass is also effective in reducing the weight of the glass plate. Thus, the density of the glass is preferably as low as possible.
Generally, an alkali-free glass of the type contains a relatively large amount of alkaline earth metal oxide. In order to lower the density of the glass, it is effective to reduce the content of the alkaline earth metal oxide. However, the alkaline earth metal oxide is a component enhancing the meltability of the glass and the reduction in content thereof decreases the meltability. The decrease in meltability of the glass results in easy occurrence of internal defects such as seeds and stones. The seeds and the stones present in the glass are serious defects as a glass plate for a display because transmission of light is prevented. In order to suppress such internal defects, the glass must be melted at a high temperature for a long time.
However, melting at a high temperature increases the load upon a glass melting furnace. For example, a refractory such as alumina or zirconia used as a material of the furnace is more heavily eroded as the temperature is higher. In this event, the life of the furnace is shortened. In addition, appliances and equipments which can be used at a high temperature are restricted and relatively high in cost. Furthermore, in order to continuously keep the interior of the furnace at a high temperature, the running cost is high as compared with a glass meltable at a low temperature. Thus, melting at a high temperature is very disadvantageous in production of the glass. It is therefore desired to provide an alkali-free glass meltable at a low temperature.
As another important requirement, the glass plate of the type must have a thermal shock resistance. At an end face of the glass plate, microscopic flaws and cracks are present even if chamfering is performed. In case where thermal tensile stress is concentrated to the flaws and the cracks, the glass plate may possibly be broken. The breakage of the glass plate decreases an operation rate of a production line. In addition, fine glass particles scattered upon breakage may be adhered to another glass plate to cause a line interruption or a patterning error in an electronic circuit to be formed thereon.
The recent development of the TFT-LCD is addressed not only to a wider screen and a lighter weight but also to the improvement in performance, such as a higher definition, a higher-speed response, and a higher aperture ratio. Most recently, for the purpose of improving the performance of the LCD and reducing the weight, development of a polycrystal silicon TFT-LCD (p-Si•TFT-LCD) is energetically carried out. An existing p-Si•TFT-LCD requires a production process temperature as very high as 800° C. or more. Such a high production process temperature allows the use of a silica glass plate alone. However, the production process temperature is lowered to a level of 400 to 600° C. as a result of the recent development. Therefore, an alkali-free glass plate is brought into use for production of the p-Si•TFT-LCD, like in an amorphous silicon TFT-LCD (a-Si•TFT-LCD) presently produced in a large amount.
As compared with a production process of the a-Si•TFT-LCD, a production process of the p-Si•TFT-LCD includes more heat treating steps. The glass plate is repeatedly subjected to rapid heating and rapid cooling so that thermal shock upon the glass plate is greater. As described above, the glass plate is increased in size. Consequently, the glass plate is susceptible to temperature distribution. In addition, occurrence of microscopic flaws and cracks at the end face of the glass plate may be caused at a high probability. This means that the glass plate may be broken in the heat treating steps at a high probability. One of the most basic and effective approaches to solve the above-mentioned problem is to reduce thermal stress resulting from the difference in thermal expansion. To this end, a glass having a low coefficient of thermal expansion is required. If the difference in thermal expansion between the glass and a material of a thin film transistor (TFT) is increased, the glass plate will be warped. Therefore, it is required for the glass to have a coefficient of thermal expansion approximate to that (about 30-33×10−7/° C.) of the TFT material such as p-Si.
The production process temperature of the p-Si•TFT-LCD is recently lowered as described above but is yet considerably high as compared with the production process temperature of the a-Si•TFT-LCD. If the glass plate has a low heat resistance, small shrinkage in dimension called heat shrinkage occurs when the glass plate is exposed to a high temperature of 400 to 600° C. during the production process of the p-Si•TFT-LCD. The heat shrinkage may possibly cause a pixel pitch error of the TFT to result in a display defect. If the glass plate has a lower heat resistance, the glass plate may be deformed and warped. Furthermore, a pattern error may possibly be caused due to the heat shrinkage of the glass plate in a liquid crystal producing step such as a film deposition step. In view of the above, the glass excellent in heat resistance is required.
On the surface of the glass plate for the TFT-LCD, a transparent conductive film, an insulating film, a semiconductor film, a metal film, and the like are deposited and various circuits and patterns are formed by photolithography etching (photo-etching). In the film deposition step and the photo-etching step, the glass plate is subjected to various kinds of heat treatments and chemical treatments.
Therefore, if the glass contains alkali metal oxides (Na2O, K2O, Li2O), alkali ions are diffused in a deposited semiconductor material during the heat treatments to deteriorate film characteristics. Accordingly, it is required for the glass to contain substantially no alkali metal oxide and to have a chemical resistance such that no deterioration is caused by various chemicals, such as acids and alkalis, used in the photo-etching step.
Generally, a TFT array process comprises a repetition of a film deposition step, a resist pattern forming step, an etching step, and a resist removing step. As an etchant, use may be made of a variety of chemical solutions, for example, a phosphoric acid solution for etching of an Al- or Mo-based film, an aqua regia (HCl+HNO3) solution for etching of an ITO film, a buffered hydrofluoric acid (BHF) solution for a SiNx or a SiO2 film. In order to save the cost, the etchant is not disposed of after single use but is recycled through a liquid circulating system.
If the glass plate has a low chemical resistance, the glass plate reacts with the etchant in the etching step to produce a reaction product which chokes or clogs a filter in the liquid circulating system. In addition, nonuniform etching causes haze of the surface of the glass. Furthermore, change in components of the etchant makes an etching rate unstable. Thus, various problems may possibly arise. In particular, a hydrofluoric acid solution such as BHF heavily erodes the glass plate and, therefore, tends to cause the above-mentioned problems. Accordingly, the glass plate is required to be excellent particularly in BHF resistance.
As regards the chemical resistance of the glass plate, it is important not only to be small in erosion but also to cause no change in appearance. Specifically, the small erosion of the glass plate by the chemical solution is very important in the sense of preventing contamination of the chemical solution and clogging of the filter by the reaction product during the etching step. As a display substrate for which a light transmittance is important, it is an indispensable characteristic that any change in appearance of the glass, such as haze or roughness, is not caused by the chemical treatment. The results of evaluation of the erosion and the change in appearance are not necessarily coincident particularly with respect to the BHF resistance. For example, those glasses exhibiting the same erosion may cause or may not cause the change in appearance after the chemical treatment, depending upon the compositions.
In the meanwhile, the glass plate for the TFT-LCD is mainly formed by the down-draw process or the float process. The down-draw process includes the slot down-draw process and the overflow down-draw process. The down-draw process is advantageous in view of the reduction in cost because the glass plate formed in this method need not be polished. However, in case where the glass plate is formed by the down-draw process, the glass tends to be devitrified. Therefore, the glass must have an excellent devitrification resistance.
presently, as the glass plate for the TFT-LCD, 1737 glass manufactured by Corning Incorporated and OA-10 glass manufactured by Nippon Electric Glass Co., Ltd., and so on are commercially available. These glasses have a strain point of about 650° C. and are therefore excellent in heat resistance. However, each glass is high in coefficient of thermal expansion and density. Therefore, if it is used as the glass plate for the p-Si•TFT-LCD, the thermal shock resistance is insufficient. In addition, the glass is heavy. This makes it difficult to increase the size and to reduce the thickness.